Modern documents are often defined using a markup language such as XML or HTML and described as markup documents. Modern documents may also be classified as fixed layout documents or adaptive documents. Adaptive documents re-layout the contents of the document based on the desired overall size of the content, such as a single sheet of paper or the size of a display window. Fixed layout documents describe where each discrete unit of a page's content, such as an image or a single run of text, is positioned on the page. Fixed layout documents are also frequently markup documents. The XML Paper Specification (XPS) describes one such document format (the XPS Document format) that is a fixed layout document and also a markup document.
Fixed layout markup document formats, such as the XPS Document format, define a document as a set of markup elements. Individual markup elements define portions of document content and a location where the content should be displayed within the document. Fixed layout markup document formats do not natively handle structural information relating to the document markup; such as to recognize that a particular portion of the markup represents a portion of a paragraph, a cell of a table, a header, or a footer, among others. Accordingly, fixed layout markup document formats do not readily establish a structural context for document markup.